Interface Level

Parameters

Descriptive string. This is obsolete and should be NULL. (Non-NULL strings are legal, but they are a waste of kernel memory.)

type

Variant type of the semaphore. Currently, only SEMA_DRIVER is supported.

arg

Type-specific argument; should be NULL.

Description

These functions implement counting semaphores as described by Dijkstra. A semaphore has a
value which is atomically decremented by sema_p() and atomically incremented by
sema_v(). The value must always be greater than or equal to zero. If
sema_p() is called and the value is zero, the calling thread is
blocked until another thread performs a sema_v() operation on the semaphore.

Semaphores are initialized by calling sema_init(). The argument, val, gives the initial
value for the semaphore. The semaphore storage is provided by the caller but
more may be dynamically allocated, if necessary, by sema_init(). For this reason, sema_destroy()
should be called before deallocating the storage containing the semaphore.

The sema_p_sig() function decrements the semaphore, as does sema_p(). However, if the semaphore
value is zero, sema_p_sig() will return without decrementing the value if a
signal (that is, from kill(2)) is pending for the thread.

The sema_tryp() function will decrement the semaphore value only if it is greater
than zero, and will not block.

Return Values

0

sema_tryp() could not decrement the semaphore value because it was zero.

1

sema_p_sig() was not able to decrement the semaphore value and detected a pending signal.

Context

These functions can be called from user, interrupt, or kernel context, except for
sema_init() and sema_destroy(), which can be called from user or kernel context only.
None of these functions can be called from a high-level interrupt context. In
most cases, sema_v() and sema_p() should not be called from any interrupt context.

If sema_p() is used from interrupt context, lower-priority interrupts will not be serviced
during the wait. This means that if the thread that will eventually perform
the sema_v() becomes blocked on anything that requires the lower-priority interrupt, the system
will hang.

For example, the thread that will perform the sema_v() may need to first
allocate memory. This memory allocation may require waiting for paging I/O to complete,
which may require a lower-priority disk or network interrupt to be serviced. In
general, situations like this are hard to predict, so it is advisable to
avoid waiting on semaphores or condition variables in an interrupt context.